The production of biogas is of growing interest as fossil-fuel reserves decline. However, there exists a dearth of literature on the design considerations that would lead to process optimization in the development of anaerobic digesters aimed at creating useful commodities from the ever-abundant municipal solid-waste. Consequently, this paper provides a synthesis of the key issues and analyses concerning the design of a high-performance anaerobic digester. 2007 Elsevier Ltd. All rights reserved.

The processing of refuse was usually undertaken to reduce the pollution potential and volume for ease of handling and disposal. This perspective has since been adjusted to include the transformation of the waste, which was hitherto unwanted, into useful end-products. This is the case with municipal solid-waste (MSW), which has a high potential for the generation of biogas (and hence energy) when subjected to anaerobic digestion [1]. So batch and continuous anaerobic-digesters have been designed for the treatment of MSW to yield biogas [2]. A preliminary design procedure includes an investigation of the properties of the refuse, with a view to establishing appropriate principles and considerations for the design of the digesters. Coupled with other relevant information from the literature, this leads to the formulation of design criteria. This report

Applied Energy 85 (2008) 430–438 w.elsevier.com/locate/apenergy integrates these technical with economic considerations to form basis for the design of anaerobic digesters for the release of biogas.

1.1. Biogas production

When organic refuse decays, it does so in the presence or absence of air (and hence oxygen) and is referred to as aerobic or anaerobic decomposition respectively. This decomposition could be naturally occurring or may be artiﬁcially induced, under controlled conditions. In either case, there are several by-products as shown in Fig. 1, which has been adapted from Ref. [3].

One of the end-products of anaerobic decay is biogas, which is produced naturally from decay under water or in the guts of animals, and artiﬁcially in airtight digesters. Itodo and Phillips [4] described biogas as ‘‘a methane-rich gas that is produced from the anaerobic digestion of organic materials in a biological-engineering structure called the digester’’. This deﬁnition suggests that biogas is only produced artiﬁcially, but this is not the case. It is believed that the scope of their deﬁnition may perhaps have been limited by their comparison of artiﬁcial production-processes, thus ignoring the natural occurrence of biogas. However, Itodo and Phillips are not alone in this way of deﬁning biogas. GEMET [5] states that biogas is ‘‘gas rich in methane, which is produced by the fermentation of animal dung, human sewage or crop residues in an air-tight container’’.

The decomposition of organic matter in the absence of air could be elicited by the use of physical or chemical processes at high temperature and/or pressure, or the use of microorganisms at near ambient temperature and atmospheric pressure; the preferred method being dependent on the relative polluting impacts on the environment. However, irrespective of the method used, gas is produced; it is referred to as biogas if generated as a result of the action of microorganisms on the organic wastes [6]. This is why biogas – see Table 1 – is now deﬁned as ‘‘a by-product of the biological breakdown, under oxygen-free conditions of organic wastes such as plants, crop residues, wood and bark residues, and human and animal manure — and is known by such

other names as swamp gas, marsh gas, ‘will o’ the wisp’ or gobar gas’’ [7], digestion gas [8], natural gas [9], landﬁl gas (LFG), and sewage gas [10]. The gas is colourless, relatively odourless and ﬂammable: it is also stable and non-toxic. It burns with a blue ﬂame and has a caloriﬁc value of 4500–5000 kcal/m3 when its methane content ranges from 60% to 70% [1].

1.1.1. Sources of biogas

The generation of biogas has traditionally been from feedstocks such as ‘‘livestock farm-waste (e.g. various manures, slurries and waste waters) and agro-industrial waste (from abattoirs, wineries, vegetable-processing plants, etc.)’’ [12]. This is why biogas is also described as the fuel produced through anaerobic fermentation of manure and vegetable matter in digesters, or the fermentation of animal dung, human sewage or crop residues in an airtight container [5]. Hence, the general belief is that liquid-manure systems work best for anaerobic digestion in the production of biogas. However, this is not so, except that the generation of biogas was indeed ﬁrst associated with liquid wastes and sludge. So Kiely [13] explained that anaerobic digestion is used worldwide for the treatment of industrial, agricultural and municipal waste-water and sludge: he also noted that, in recent years, it has also been applied for the treatment of municipal solid-wastes. Hence Vassiliou [12], after successfully generating biogas from wastes of raw manure plus wash-water from large livestock-farms, and the wastes from food and drink industries, explained that the second stage of any project should be to generate biogas from the organic components of source-separated municipal solid-wastes (MSWs).

1.1.2. Uses of biogas

Biogas is increasingly becoming an attractive source of energy in many nations of the world. For example, the Finnish magazine Suomen iuonto reported that the gas is used to fuel a car owned by a farm owner. Sweden too has a similar story as many city-buses are powered by biogas, and some gas stations there oﬀer biogas in addition to other fuels. In fact, all over the world, biogas has been variously used for heating purposes and/or electricity generation. For example, in the UK, Xuereb [10] reported that, although the use of biogas for electricity generation was still at an experimental stage, it already accounts for about 0.5% of the total electricityoutput; and biogas fuels account for about 1% of US electricity generation, while achieving a climate-change beneﬁt equivalent to reducing CO2 emissions in the electricity sector by more than 10%. Biogas is also presently used in India, China, Taiwan, Brazil, Singapore, etc. Tchobanoglous and Burton [14] stated that, in large plants, digester gas may be used as a fuel for the boiler and internal-combustion engines, which are in turn used for pumping waste water, operating blowers and generating electricity. Despite the heating-and-electricity generation uses of biogas, in addition, the residues of such biogas production can be used as low-grade fertilizers. Xuereb [10] enumerated the characteristics of biogas:-

• It is ﬂammable, potentially explosive and a readily controllable source of energy. • Its use helps to reduce the amount that would otherwise be released naturally into the atmosphere, and so reduces the excessive greenhouse-eﬀect.

• Although on burning biogas, carbon dioxide is released, it is not considered as a net contributor to the global carbon-dioxide level because it originated from plants, which have absorbed it from the atmosphere. Hence this carbon dioxide does not make a net contribution to the ‘greenhouse eﬀect’.

Table 1 Composition of biogas

Constituent Composition

Oxygen (O2) Traces

432 A. Hilkiah Igoni et al. / Applied Energy 85 (2008) 430–438

• The harnessing of biogas also helps to minimize the unpleasant decomposition smels produced in landﬁl sites because, otherwise, these gases would be released directly into the atmosphere. Hence, especially where landﬁlls are situated close to inhabited areas, the harnessing of LFG makes landﬁlls slightly more socially acceptable.

1.2. Municipal solid-waste

Generally, such refuse is regarded as useless material that is unwanted and therefore discarded. The New

Edition Concise English-Dictionary [15] explains that ‘‘waste’’ is ‘‘anything or anyone rejected as useless, worthless, or in excess of what is required’’. But Byrne [16] was more comprehensive with his description of waste, when he stated that waste is material, which has no direct value to the producer and so must be disposed of. This could be why Bailie et al. [17] insist that ‘‘for practical purposes, the term waste includes any material that enters the waste-management system’’. A waste-management process and pertinent system are an organized programmes and central facility respectively established, not only, for the ﬁnal disposal of waste but also for recycling, reuse, composting and incineration. From the foregoing, it is apparent that materials enter a waste-management system when no one who has the opportunity to retain them wishes to do so.

Wastes are usually classiﬁed, as gaseous-, liquid- or solid-wastes, depending on their phase. Materials which fall within the solid-waste category form the subject of the present study. Bailie et al. [17] deﬁned solid-wastes to include all refuse materials that are not hazardous, liquid wastes or atmospheric emissions. While Kiely [13] speciﬁed solid wastes as including those from human and animal activities, and include liquid wastes like paints, old medicines, spent oils, etc. It is possible to have solid-waste intermixed with liquid waste. Nevertheless, in whichever manner the waste occurs, this study considers solid waste as largely non-ﬂowing, which makes its handling and management relatively diﬃcult, compared with those for liquid and gaseous wastes [18]. Its non-ﬂowing nature requires its continual retention at the site of generation or deposition, until it is removed for disposal. Consequently, solid waste poses many environment problems, including oﬀensive odours; obstruction of traﬃc ﬂow; as well as blocking of waterways and drains, so leading to ﬂooding, environmental degradation and pollution of the atmosphere, with the concomitant unfavourable eﬀects on public health, etc. The common way of describing the solid-waste menace is usually with respect to its place of generation or point of origin.

Municipal solid-waste (MSW) is deﬁned as all waste collected by private and public authorities from domestic, commercial and some industrial (non-hazardous) sources. Furthermore, Kiely [13] and Bailie et al. [17] posited that MSW in particular comprises small and moderately sized solid-waste items from houses, businesses and institutions. Also Byrne [16] stated that municipal waste is that generated from urban areas, particularly houses and shops.

2. Digester-design considerations

In the design of a digester suitable for the biodegradation and indeed stabilization of MSW, with the attendant production of biogas, several factors are considered, such as the type of waste, the rate of waste generation and local environmental conditions, like the ambient temperature.

2.1. Type of digester

A variety of digester types exists for the anaerobic treatment of organic wastes. The selected type depends on operational factors, including the nature of the waste to be treated, e.g. its solid content. The Oregon State Department of Energy [8] in its classiﬁcation of types of digester explains that ‘a covered lagoon digester’ is used for liquid-manure of less than 2% solids; ‘a complete-mix digester is suitable for manure that is 2–10% solids’; and ‘plug-ﬂow digesters are suitable for ruminant animal excreta having solid concentrations of 1– 13%’’. The type and solid contents of the waste they considered were such that the wastes are capable of ﬂowing on their own, or forming slurries with water and eventually ﬂowing and so could be used in a continuous operation.

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MSW is predominately solid and non-ﬂowing. Hence Kahaynian et al. [19], as reported by Kiely [13], suggested the use of ‘high-solids digesters’. This was expounded by Hobson et al. [6] when they posited that solidstate digestion, though still largely theoretical with only some small units having being built, is always a batch process. But continuous-ﬂow digesters, which are by nature low-solids digesters, have been employed to generate methane from human, animal and agricultural wastes, and from the organic fraction of MSW [20]. This process always required more water to be added to the waste in order to get it to ﬂow.

Hobson et al. [6] reported an experimental digestion of domestic garbage diluted with sewage to a total solids concentration of between 5 and 7%. Fig. 2 shows the ﬂow process in a low-solids digester using MSW, as described by Tchobanoglous et al. [20]. The authors [6] maintained that stirred – tank digesters can deal with slurries of about 3–10% total solids (TS), much of which are suspended solids. In their analysis of what they described as ‘solid feedstock’, including green-vegetable matter of about 20% solid concentration, they explained that, if these materials are to be used as feed for a stirred-tank digester, then they will have to be made into slurries. According to their experience, a slurry of about 10% TS is the maximum that can be pumped and piped even if the particle size is small, and 7–8% TS may be the maximum which can be handled by smaller pumps and pipelines.

2.2. Temperature control

The temperature of MSW aﬀects the success of the digestion process, as the activities of the anaerobes causing waste decomposition are temperature dependent. The optimal temperature ranges are the mesophilic, namely 30–38 C, and the thermophilic 4–57 C [14], respectively. The rate of decomposition and gas production is sensitive to temperature, and, in general, the process becomes more rapid at high temperatures [3]. Despite this ‘thermophilic beneﬁt’, the digestion process becomes increasingly unstable with rising temperature, and requires higher rates of heat inputs, and produces poorer-quality supernatant containing larger quantities of dissolved solids [14]. However, Kiely [13] insists that most digesters now operate at mesophilic temperatures, for which good stability and gas production occur. In addition, Tchobanoglous et al. [21] posited that reactor temperatures between 25 C and 35 C are generally the preferred optima to support biological-reaction rates yet provide a more stable treatment. Mattocks [7] previously noted that choosing the schredder municipal solidwaste ferrous materialsheavy fractionother energy conversion units blending-mixing tank chemical or sewage sludge feed atmosphere liquid-air separator solids to landfill digester 60 oc filtrate slurry pump light fraction magnetic separator air classifier